In complementary metal oxide semiconductor (CMOS) integrated circuits, dynamic power consumption is equal to the capacitance of the integrated circuit times the frequency of operation times the square of the voltage, or P=CV2f. In order to reduce power consumption, modern microprocessors have adopted dynamic power management using “P-states”. A P-state is a voltage and frequency combination. An operating system (OS) determines the frequency required to complete the current tasks, such as real-time execution of an application program, and causes an on-chip power state controller to set the clock frequency accordingly. For example, if on average the microprocessor is heavily utilized, then the OS determines that the frequency should be increased. On the other hand if on average the microprocessor is lightly utilized, then the OS determines that the frequency should be decreased. The available frequencies and corresponding voltages necessary for proper operation at those frequencies are stored in a P-state table. As the operating frequency increases, the required power supply voltage also increases, but because power is related to the square of the voltage, it is important to keep the voltage low while still ensuring proper operation.
When populating the P-state table, integrated circuit designers not only determine the voltage that will ensure proper operation at a given frequency, but also provide sufficient margin so that the microprocessor operates correctly even under so-called “worst case” conditions. This margin is known as the “guardband”. However setting the guardband for “worst case” conditions means that most of the time the voltage is higher than necessary and thus the microprocessor consumes additional power.
In the following description, the use of the same reference numerals in different drawings indicates similar or identical items. Unless otherwise noted, the word “coupled” and its associated verb forms include both direct connection and indirect electrical connection by means known in the art, and unless otherwise noted any description of direct connection implies alternate embodiments using suitable forms of indirect electrical connection as well.